Enhancement, stabilization and metallization of porous silicon

ABSTRACT

A post-etch treatment for enhancing and stabilizing the photoluminescence (PL) from a porous silicon (PS) substrate is outlined. The method includes treating the PS substrate with an aqueous hydrochloric acid solution and then treating the PS substrate with an alcohol. Alternatively, the post-etch method of enhancing and stabilizing the PL from a PS substrate includes treating the PS substrate with an aqueous hydrochloric acid and alcohol solution. Further, the PL of the PS substrate can be enhanced by treating the PS substrate with a dye. Furthermore, the PS substrate can be metallized to form a PS substrate with resistances ranging from 20 to 1000 ohms.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to copending U.S. provisionalapplication entitled, “Porous Silicon for Sensor Applications:Enhancement, Stabilization and Metallization,” having Serial No.60/192,845, filed Mar. 29, 2000, which is entirely incorporated hereinby reference.

TECHNICAL FIELD

[0002] The present invention is generally related to porous siliconsubstrates and, more particularly, is related to a method forphotoluminescence enhancement, photoluminescence stabilization, and themetallization of porous silicon substrates.

BACKGROUND OF THE INVENTION

[0003] High surface area porous silicon (PS) substrates formed in waferscale through electrochemical (EC) etching fall into two groups. PSsubstrates fabricated from aqueous electrolytes consists of highlybranched nonporous substrates while PS substrates fabricated from ananoqueous electrolyte is comprised of open and accessible macroporoussubstrates with deep, wide, well-ordered channels.

[0004] High-surface area substrates formed in wafer scale throughetching display a visible photoluminescence (PL) upon excitation (PLE)with a variety of visible and ultraviolet light sources. Thisroom-temperature luminescence has attracted considerable attentionprimarily because of its potential use in the development ofsilicon-based optoelectronics, displays, and sensors.

[0005] Although the PL is thought to emanate from regions near the PSsubstrate surface, the origin of the PL is the source of somecontroversy as the efficiency and wavelength range of the emitted lightcan be affected by the physical and electronic properties of thesurface, the nature of the etching solution, and the nature of theenvironment into which the etched sample is placed. Given this range ofparameters, it is surprising that, with few exceptions, PL spectra arereported for PS substrates formed in dilute aqueous HF solutions, thathave already been dried in air or more inert environments following etchand rinse treatments. These ex situ samples, while providing spectralinformation, do not indicate the evolution of the PS substrates, andthus, they do not indicate means by which it might be modified andenhanced during or following the etch treatment.

[0006] An existing problem in fabricating PS devices rests withestablishing electrical contact to the PS substrates. Another problemwith PS includes the relatively long excited-state lifetime associatedwith the PS substrate PL. A further problem includes the relatively lowPL quantum yield and the instability of the PL from PS substrates.

[0007] Thus, a heretofore unaddressed need exists in the industry toaddress the aforementioned deficiencies and inadequacies.

SUMMARY OF THE INVENTION

[0008] An embodiment of the present invention provides for a post-etchtreatment method of enhancing and stabilizing the PL from a PSsubstrate. The method includes treating the PS substrate with an aqueoushydrochloric acid (HCl(H₂O)) solution and then treating the PS substratewith an alcohol. Another exemplary embodiment provides a post-etchmethod of enhancing and stabilizing the PL from a PS substrate, whichincludes treating the PS substrate with an HCl(H₂O) and alcoholsolution.

[0009] Still another embodiment provides a post-etch method formetallizing a PS substrate in an electroless environment. The methodincludes treating the PS substrate with an HCl(H₂O) solution and thentreating the PS substrate with an alcohol, or alternatively, treatingthe PS substrate with a hydrochloric acid/alcohol solution.Subsequently, the PS substrate is treated a hydrazine solution to removefluorides from the PS substrate. Next, a metal-containing electrolesssolution is introduced to the PS substrate. Thereafter, the PS substrateis illuminated with a light source at wavelengths less than about 750nanometers to cause PL of the PS substrate. Then, the metal from themetal-containing solution is induced by the PL to reduce onto the PSsubstrate (e.g. metallization).

[0010] A further embodiment provides for a post-etch method of enhancingPL from a PS substrate by treating the PS substrate with a dye. The dyecan be selected from the group including, but not limited to3,3-diethyloxadicarbacyamine iodide; Rhodamine dye compounds;Fluorocein; and dicyanomethylene (DCM) dye compounds.

[0011] Still a further embodiment provides for a metallized PSsubstrate. The PS substrate can be metallized with copper, silver, orother appropriate metal and can have a resistance of about 20-100 ohm.

[0012] Other systems, methods, features, and advantages of the presentinvention will be or become apparent to one with skill in the art uponexamination of the following detailed description. It is intended thatall such additional systems, methods, features, and advantages beincluded within this description, be within the scope of the presentinvention, and be protected by the accompanying claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013] One of a number of embodiments of the present invention includesthe treatment of PS substrates generated in an aqueous and nonaqueousetch with an HCl(H₂O) solution, which results in the stabilization andenhancement of the in situ PL of the PS substrates. More specifically,in an exemplary embodiment, in a post-etch treatment method, an HCl(H₂O)solution can be used to enhance and stabilize the PL (in situ) from a PSsubstrate. In addition, in an other exemplary embodiment, a method oftreating the PS substrate with HCl(H₂O) followed by an alcohol solution(e.g. methanol or ethanol) further enhances and stabilizes the PL (insitu and ex situ) of the substrate. A nonlimiting illustrative exampleincludes PS substrates that are treated in an aqueous hydrochloric acidand water (HCl/H₂O) solution and display a strongly enhanced in-situluminescence; however, the PL decays rapidly in an ex-situ environmentwithout treatment in alcohol, preferably a high purity alcohol. Anexemplary embodiment includes treating the PS with methanol (MeOH).Further, PS substrates treated in an HCl (H₂O)/alcohol solution (of atleast 0.2 molar (Al)) maintain their enhancement for extended periods oftime. The PS substrate may be stabilized and enhanced by the presence ofa chloride ion (Cl⁻). The treatment appears to be independent of themethod of preparing the PS substrate, implying that the chloride salttreatment largely stabilizes the surface states of the photoluminescentPS substrate. This stabilization may be demonstrated by varioustechniques including, but not limited to the following: scanningelectron micrographs (SEM), which show the profound change whichaccompanies the HCl treatment of the PS surface; Energy DispersiveSpectroscopy (EDS) which, reveals chloride incorporation into the PSsurface at strongly photoluminescent regions; and Raman scattering,which demonstrates that the PL is correlated with the creation ofamorphous structural regions. All of these testing methods indicate themanner in which the chloride salt stabilizes the PS substrate.

[0014] Another exemplary embodiment of the present invention includestreating PS substrates with a dye (e.g., 3,3′-diethyloxadicarbocyanineiodide (DODCl) and Rhodamine 700). In general, the dye should havenegligible absorption at the wavelengths of maximum absorption for thePS substrate. After a period of aging in darkness these dye-treated PSsubstrates can be pumped at about 337.1 nanometers (nm) (nitrogen laser)near the maximum in the PS absorption spectrum (far from the majorabsorption regions of the impregnating dye). Time-dependent PLhistograms indicate that the resulting PL emission rate is enhanced. Theenhancement in the PL emission rate may be attributed to an interactionbetween the surface-bound fluorophors, which characterize PS substratesand the dye. This interaction results in the creation of a distributionof PS-dye complexes, which enhance the nominal PL emission rate from theuntreated PS surface. In a preferred embodiment, the DODCl⁻ treatedsamples display PL that exceeds that of nominally prepared PS by afactor of five or more.

[0015] A further exemplary embodiment of the present invention includesthe metallization of a PS substrate. One of the existing challenges infabricating PS devices rests with establishing electrical contact to thePS substrate. In an exemplary embodiment, PS substrates are capable ofbeing metallized in a controlled manner using electroless metal-coatingsolutions and inducing the metal to plate onto the PS substrate. Anexamplary embodiment includes using an electroless metal solution, whichcan be introduced to the PS substrate after treating the PS substratewith a hydrazine solution so that subsequently the metal can bedeposited onto the PS substrate in a controlled manner. Themetal-containing solution includes, but is not limited to, any one, orall, or combination of copper, silver, gold, nickel, palladium,platinum, other metals that are commonly deposited using electrolesstechniques. This method is capable of using the “long-lived” PSsubstrate PL to enhance reduction at the PS substrates surface. This maybe accomplished by creating excited fluorophors on the PS surface toenhance interaction and reduction at the PS substrate surface. Usingthis method enables metals to readily deposit onto the PS substratewithin PS micropores and nanopores. Further, under controlled conditionsthe metallization only occurs where the PS surface is illuminated withlight from a light source. (e.g. Xenon (Xe) arc lamp, Helium-Neon (HeNe)laser, or other appropriate light source). Furthermore, the thickness ofthe metallization deposit is proportional to the time and intensity ofexposure of the PS surface to the light source.

[0016] In conventional electroless metal plating, the surface is usuallyfirst coated with palladium (Pd) metal to catalyze the depositionprocess. For purposes of this disclosure, the addition of a catalyst tothe metal plating process is considered to be operating under catalyticconditions. However, embodiments of the present invention do not requirean additional catalyst, which, for purposes of this disclosure, meansthat the method is performed under “non-catalytic conditions.” Indeed,in the method of the present invention, The illuminated PS surfaceitself is catalyzing the deposition. Further, localized heating is notpromoting the deposition; rather the metal deposition occurs when the PSsubstrate is illuminated at wavelengths less than about 750 nm,consistent with its bandgap.

[0017] A further exemplary embodiment of the present invention includesthe metallization of a PS substrate to produce a low electricalresistance metallized PS substrate that has a resistance from about 20ohms to about 1000 ohms. Another embodiment includes metallized PSsubstrates with resistances between about 20 ohms and about 100 ohms.Still a further embodiment includes metallizated PS substrates withresistances between about 20 ohms and about 60 ohms.

[0018] A. HCl/(H₂O) Alcohol Enhancement of Porous Silicon Substrates

[0019] As discussed above, an exemplary embodiment of the presentinvention includes a method and system of treating PS substrates with anHCl/(H₂O) solution to enhance and stabilize the PL of the PS substrates.PS substrates treated in an HCl/(H₂O) Solution display a stronglyenhanced in situ PL. PS substrates treated in an HCl/(H₂O) alcoholsolution (e.g. at least 0.2 M) display enhanced in situ and ex situ PLand can maintain enhancement for time periods on the order of years.Another exemplary embodiment includes treating the porous siliconsubstrates with an HCl/(H₂O) solution (e.g. at least 0.2 M) thensubsequently treating the PS substrates with an alcohol. This embodimentalso enhances and stabilizes the in situ and ex situ PL of the PSsubstrate.

[0020] More specifically, the post-etch method of enhancing andstabilizing the PL of a PS substrate includes treating the PS substratewith an HCl/(H₂O) solution. The PS substrate includes, but is notlimited to a microporous framework upon which is superimposed ananoporous layer. The HCl/(H₂O) solution is at least 0.2 M. In oneexemplary embodiment, the HCl/(H₂O) solution includes an alcohol.Alcohols that can be used include, but are not limited to, ethanol,methanol, other appropriate alcohols for treating PS substrates, andcombinations thereof. In another exemplary embodiment, the PS substrateis treated with the HCl/(H₂O) solution, then subsequently treated withan alcohol (e.g., ethanol, methanol, etc.) This method of treatmentenhances the in situ and ex situ PL.

[0021] Chloride-ion stabilization appears independent of the method ofpreparing the PS substrates, implying that the chloride salt treatmentlargely stabilizes the surface constituency of the photoluminescent PSsubstrate. This can be demonstrated by scanning electron micrographs,which show the change that accompanies the HCl treatment of the PSsubstrate surface. Further, energy dispersive spectroscopy revealschloride incorporation into the PS surface at strongly PL regions.Furthermore, Raman scattering demonstrates that the PS substrate PLenhancement is correlated with the creation of amorphous structuralregions. In conjunction with detailed quantum-chemical modeling,time-dependent histograms obtained for the HCl-treated systems indicatethat the resulting PL, initiated through the pumping of the HCl-modifiedsurface, displays the manifestation of a significant surfaceinteraction. This interaction might result in the formation of bothchlorosilanones and chlorsilylenes. In addition, the hydrogen cation(H⁺) may play a role in the stabilization of the silanol-based featuresof the PS substrate surface both as a contribution to the flourophorformation and by decreasing the hydroxyl (OH⁻) concentration insolution.

EXAMPLE 1

[0022] The following is a non-limiting illustrative example of anembodiment of the present invention that is described in more detail inGole, et al., Phys. Rev. B, 61, 5615 (2000); Gole, et al., J. Phys.Chem. 101, 8864 (1997); Gole, et al. Phys. Rev. B, 62, 1878 (2000),which all are herein incorporated by reference. This example is notintended to limit the scope of any embodiment of the present invention,but rather is intended to provide specific experimental conditions andresults. Therefore, one skilled in the art would understand that manyexperimental conditions can be modified, but it is intended that thesemodifications are within the scope of the various embodiments of thisinvention.

[0023] Single crystal <100> boron doped silicon wafers withresistivities of about 50-100 ohm cm were used in the current study.Both highly branched nanoporous and hybrid nanoporous coveredmicroporous PS substrate samples were fabricated in an electrochemicalcell constructed from high-density polyethylene. The working electrodewas attached to the back of a p-type silicon wafer <100> (aluminum(Al)coated) and the counter electrode corresponded to a platinum (Pt)foil placed in solution. The cell was sealed to the front of the wafer,using a clamp, as about a 1 centimeter squared (cm²) section of thewafer made contact with the solution. A magnetic stir-bar was used toprevent the build-up of hydrogen at the surface of the silicon. Theelectrochemical etching current was supplied by anPotentiostat/Galvanostat. The nanoporous samples were etched in anaqueous 25% hydrofluoric acid (HF) in methanol solution while the hybridsamples were etched in a solution of 1 M H₂O, 1M HF, and 0.1Mtetrabutylammonium percholate (TBAP), all in acetonitrile. The aqueousetched samples were etched at a current density ranging from about 5 to6 miliamp per square centimeter squared (mA/cm²) for about 50 to 75minutes while the hybrid samples were etched with a current density ofabout 6 mA/cm² for between 50 and 75 min. Using this latter procedure,pores approximately 1 to 2 μm wide by about 10 μm deep were formed, andwell covered by a coating of nanoporous silicon.

[0024] Single-crystal <100>, boron-doped silicon wafers (substrate) ofresistivity ranging from about 1 to about 50 ohm cm were also etched inan aqueous HF solution. For several of the experiments (about 20%concentration of HF in methanol), a 300-nm thin film of aluminum wassputtered onto the backside of the wafers. Electrical connections weremade to the wafers by connecting a wire to the thin film of aluminumusing conductive paint. The wire and aluminum film were then coveredwith a layer of black wax, leaving only the front surface of the siliconexposed to the etching solution. Both the wired silicon-wafer as oneelectrode and a platinum wire as a counter-electrode may be connectedthrough a Teflon™ cap, which was tightly fitted to a cuvette containingthe etch solution. Ohmic contacts were made to the wafer by connecting awire to the thin film of aluminum using conductive paint. Etchingcurrents ranged from about 2 to 30 mA/cm² but the samples consideredhere were usually etched at 8 mA/cm² for 10 minutes.

[0025] These aqueous HF-etching procedures are capable of leading to theformation of a “nanoporous” surface on the silicon wafer. However, inorder to obtain a thicker nanoporous coating and increase the samplingvolume, Si<100> samples were treated in a 25% HF in methanol solution ata current density of about 14 mA/cm² for a period of about 30 min. Inall cases, the prepared aqueous samples were either treated directly insolution or washed in spectral quality methanol and dried in air. Theywere then subsequently transferred to a crucible containing eithermethanol or the postetch chloride solution of interest.

[0026] Samples prepared in about 20% HF/MeOH aqueous etching solution,once rinsed in a combination of doubly deionized water and methanol orethanol and allowed to dry, yield a significant ex situphotoluminescence. If a cleaned sample is immediately placed in doubledeionized water, a gradual rise in the in situ PL intensity at 620 nmcan be observed. Similarly, if such a cleaned sample is placed in anultra high purity low molecular weight alcohol (e.g. methanol andethanol) solution, the in situ PL will slowly increase and graduallydiminish on the time scale of several hours. However, it is to be notedthat this behavior is in sharp contrast to the effect of the alcohols incombination with HF: this combination rapidly quenches the PL withinseveral minutes.

[0027] The post-etch treatment of the PS substrates with an aqueous HClsolution and the enhancement and stabilization is now discussed. A PSsample prepared by aqueous etch is first etched in a methanol/20% HF [6mol./l HF in MeOH(aq)] solution for 10 min., washed with methanol, driedin air, and then placed first in a solution of doubly deionized waterand then dilute 6M HCl. The intensity of the PL, excited by a nitrogen(N) laser, increases somewhat upon removal from the HF etch solution andagain increases gradually as the sample is placed in water. However,upon adding HCl the orange-red PL intensity increases significantly andremains constant over the time period (up to about 3.5 h) in which thesample is present in the HCl solution. Within the time frame of theseprocesses, the wavelength-dependent spectral profile of the PL emissionspectrum appears to have been altered only slightly from the time it wasremoved from the HF etching solution and dried in air through the periodin which the sample remained in the HCl solution. The introduction of ahigh concentration of HCl has stabilized the PS photoluminescence.

[0028] In contrast to the stabilizing effect that 6M hydrochloric acidhas on the PS substrates surface, a 2.75M hydrogen iodide (HI) solutionalmost completely quenches the PL. The effect of a 4.5M hydrogen bromide(HBr) solution is intermediate. The effect of the HI solution can beattributed to the strong quenching of surface-bound flurophors resultingfrom the formation of I₂ and I₃ in an oxidizing acidic environment. Asimilar effect also occurs with bromine, albeit to a much lesser extent.

[0029] Post-etch treatment of the PS substrate in a methanol-NaClsolution is now discussed. The pronounced stabilization of the PSsubstrate photoluminescence in the 6M HCl solution focuses on the effectthat the chloride ion may have on the PL process. The introduction of anaqueous etched (20% HF/MeOH) PS substrate sample into a saturatedNaCl/MeOH solution produces a clear saturating PL emission signal. Thisis manifest in two ways. In the absence of sodium chloride (NaCl), the620-nm PL from PS substrate placed in methanol solution slowly rises,eventually peaks, and then more gradually decreases in intensity. IfNaCl is placed into this solution before the PL has reached its maximumintensity in methanol, the PL will slowly increase to a maximum thenplateau. For the introduction of the PS substrate sample into aNaCl-saturated solution in methanol, this plateau is reached over a timescale of several hours (compared to about 30 min for a 6M HCl solution).Furthermore, the photon-count level at the maximum PL intensity appearsto be considerably muted relative to the observed maximum for a PSsubstrate sample in methanol alone. In contrast, if the saturated NaClsolution is introduced to the methanol solution after the PLphoton-count level has peaked in the methanol solution, the PL is foundto plateau at an intensity corresponding closely to that at the time ofthe NaCl introduction.

[0030] Post-etch treatment of PS substrate samples in tetrabutylammoniumchloride (Cl⁻) solution is now discussed. The results observed whenplacing a prepared PS substrate sample in 6M HCl and saturated NaClsolutions certainly call attention to the potential role of the chlorideion in stabilizing the PS substrate photoluminescence. In order to studythe effect of varying chloride-ion concentrations on the PS substrateluminescence, both tetrabutylammonium perchlorate (TBAP) and HClsolutions are studied.

[0031] The PS substrates PL at 620 nm is monitored after a sample,etched in a 20% HF/MeOH solution, is washed in methanol, dried in air,and then placed in a tetrabutylammonium chloride (TBAC) in methanolsolution. The TBAC concentrations used include 0.1M, 0.2M, 0.3M, 0.4Mand 1.0M. These experiments can be used to compare directly to theNaCl/MeOH saturated solution results. The 0.1M TBAC solution leads to aPL intensity which peaks at about 4300 counts, seventy five minutes intothe bath cycle, and then it monotonically decreases to about 1000 countswithin 5 hours. The source of the PS luminescence is temporarilyenhanced, but it is not being stabilized at longer time scales. When thechloride concentration is raised to 0.2M, the photon-count levelincreases moderately as the PL intensity peaks at about 4800 counts, now150 min. into the bath cycle. The signal again monotonically decreasesto about 1000 counts within 5 hours. This trend continues for the 0.3Msolution as the PL intensity peaks at about 6500 counts, 240 min intothe run; however, despite a significant peak photon count, an eventualdrop-off the PL signal is observed. The 0.4M chloride-ion solution againdemonstrates an increased photon-count level (peak about 12000 counts).There is also a notable decrease in the rate of PL decay. This trendappears to reverse for the 1M concentrated Cl⁻ solution. Although the PSluminescence peaks on a shorter time scale, the PL peak level hasdropped to about 6000 counts and also decays at a much more rapid rate,clearly paralleling that for the 0.1M and 0.2M solutions. Thus, for thetetrabutylammonium counterion (TBA⁺), evidence is produced for a peakeffective chloride-ion concentration but no evidence is produced for anextended stabilization of the PL signal with time.

[0032] In situ stabilization in HCl solutions of varying concentrationsis now discussed. The results obtained in saturated NaCl and TBACsolutions emphasize the remarkable PL stabilization that is inherent toa PS sample bathed by a 6M HCl solution. Next, a comparison of variousconcentrations of HCl is conducted. The concentrations include 0.1M,0.2M, 0.3M, 1M, 2M, and 3M. For a 3M HCl solution in either water ormethanol, the photon count rate is comparable to that for the 6M HClsolution. The count rate is still rising after 6.5 h in dilute HCl,whereas it levels off at about 20000 counts in the 6M HCl/(H₂O) MeOHsolution after approximately 2 h. Both bath solutions demonstrate aprofound stabilizing and enhancing effect on the PS emission intensity.

[0033] As the HCl molarity decreases, there are clear subtle changes inthe in situ PL. For the 2M and 1M dilute HCl/(H₂O) solutions, the countrate is again comparable to that for the higher molarities and stillrising after 6 h; however, the count rate in the 2M HCl/MeOH solutionhas already dropped to about 14000 (although appearing reasonablystabilized) and shows an even sharper decline for the 1M solution wherea maximum intensity of about 5000 counts occurs about 2 h into the timescan and a precipitous decrease to barely 3000 counts is observed at 6h.

[0034] The HCl-water system displays a remarkable in situ enhancementand stabilization. With further dilution to 0.3M, one finds a comparablebut possibly a slightly increased enhancement of the PL intensity isobserved, which is still rising to about 25,000 counts after 6 h of PSsubstrate sample exposure to the HCl solution. At 0.2M, the HCl solutionagain displays a comparable stabilization that appears to plateau atabout 22000 counts. However, at 0.1M, the HCl solution induces a muchsmaller enhancement of the PL signal from a sample photoluminescing indoubly deionized water. Furthermore, the stabilization of the signal ismarginal as shown by a peaking at about 2625 counts approximately 4 hinto the run. The threshold for stabilization and enhancement thusappears quite dramatic.

[0035] The results shown for HCl suggest the importance of the H⁺counterion generated from the strong acid HCl as well as the chlorideion. The H⁺-ion concentration may play a role in stabilizingsilanol-based features on the PS substrate surface both as a contributorto flurophor formation and by decreasing the [OH⁻] concentration insolution. This is supported because the introduction of NaOH into thisbath solution completely quenches the PL as it significantly increasesthe hydroxyl-ion concentration.

[0036] Ex situ PL from HCl-treated PS substrates is now discussed. Thesamples that have been treated in HCl(H₂O)/MeOH and HCl/H₂O solutionsexhibit distinctly different ex situ behavior. The followingdemonstrates the different behavior. The growth of the in situ PLemission in a 1 M HCl (H₂O)/MeOH solution peaks at about 12500 counts.Upon removal from the HCl(H₂O)/MeOH bath, the ex situ PL emissionintensity continues to maintain itself for periods exceeding severalmonths. A similar treatment of the PS substrate surface in 3MHCl(H₂O)/MeOH also produces a highly photoluminescent ex situ samplewith a considerably higher long-term photoluminescent emission intensitypeaking at about 18000-20000 counts (about 620 nm). In sharp contrast,if a PS substrate sample treated in an HCl/H₂O solution andcharacterized by a PL emission intensity close to 20000 counts isremoved from the solution and dried in air, the PL emission intensitydrops to about 4000 counts at 620 nm within 24 h even decreasingprecipitously during the laser pumping and PL measurement period toabout 3000 counts. Another comparison shows long-term PL stability, fora 1M HCl/H₂O-tested sample that is immediately placed in ultra highpurity methanol. The PL scans display not only a long-term stability butalso a peak intensity considerably red-shifted (˜500 Å). The ex situ PLsignal from an HCl/H₂O solution-treated sample is almost completelyextinguished within only a few days. However, if the PS substrate sampleis rinsed in ultra high purity methanol after an in situ HCl/H₂Otreatment and allowed to remain in a methanol solution for two to threedays, the PL intensity can significantly be maintained, ex situ,indefinitely.

[0037] B. Metallization of PS Substrates

[0038] An existing challenge in fabricating PS devices rests withestablishing electrical contact to the PS substrates. An exemplaryembodiment of the present invention uses the methods of enhancement andstabilization of the PS substrates and the excited state fluorophorsthat can be created on the PS surface to enhance reduction(metallization) at the PS substrate surface. Embodiments of the presentinvention include using the excited state fluorophors, whose interactionand reducing capabilities are greatly enhanced relative to that of theirground states, to induce the deposition of the metal from ametal-containing solution onto the surface of the PS substrates. Metalions can be reduced and deposited on the PS substrates and within thepores of the PS substrates. The deposition occurs in regions of the PSsubstrates that are illuminated with light from a light source (e.g.Xenon arc lamp, HeNe laser etc.). The illumination produces PL from thePS substrates which in turn causes metallization of the PS substrates.The thickness of the deposit upon the PS substrates is proportional tothe time and intensity of exposure to the light source. Whileconventional electroless metal plating generally requires surfacecoating with Pd metal to catalyze the deposition process, exemplaryembodiments of the present invention require no catalyst to deposit themetals of interest as the illuminated PS surface is itself catalyzingthe deposition. These embodiments are therefore under non-catalyticconditions since no additional catalyst is needed to metallize the PSsubstrates.

[0039] More specifically, the post-etch method of eletrolessmetallization of PS substrates includes treating the PS substrates withan HCl(H₂O) solution. The PS substrates has a microporous framework onwhich is superimposed a nanoporous layer. The HCl(H₂O) solution is atleast 0.2M. In an exemplary embodiment, the HCl(H₂O) solution includesan alcohol. The alcohols that may be used include, but are not limitedto, ethanol, methanol, other appropriate alcohols for treating PSsubstrates, and any combination thereof In another exemplary embodiment,the PS substrate is treated with an HCl(H₂O) and alcohol solution. Thenext step, for both of the previous embodiments, includes treating thePS substrate with a hydrazine solution, which can remove fluorides fromthe porous silicon substrate. Thereafter, the PS substrate is introducedto a metal-containing solution. The metal-containing solution includes,but is not limited to, copper, silver, nickel, gold, platinum,palladium, other appropriate electroless metals, and combinationsthereof. Thereafter, the PS substrate is illuminated with a light sourceof less than a wavelength of 750 nanometers. The light source includes,but is not limited to, a Xenon arc lamp, HeNe laser, or any otherappropriate light source producing light of wavelength less than 750nanometers for the metallization process. The illumination of the PSsubstrates causes the metal of the metal-containing solution to bereduced upon the surface and in the micropores and nanopores of the PSsubstrates. The PS substrate can also include a pattern or mask suchthat only portions of the PS substrate surface is metallized. Themetallized PS substrate fabricated is capable of having resistances inthe range of 20-1000 ohms. In addition, the metallized PS substrate canhave a resistance in the range of 20-500 ohms. A preferred embodimentincludes a metallized PS substrate that has a resistance in the range of20-60 ohms.

EXAMPLE 2

[0040] The following is a non-limiting illustrative example of anembodiment of the present invention which is discussed in more detail inGole, et al., J. Electro. Soc., 147, 3785 (2000), which is hereinincorporated by reference. This example is not intended to limit thescope of any embodiment of the present invention, but rather is intendedto provide specific experimental conditions and results. Therefore, oneskilled in the art would understand that many experimental conditionscan be modified, but it is intended that these modifications are withinthe scope of the embodiments of this invention.

[0041] As discussed for example 1, single crystal <100> boron dopedsilicon wafers with resistivities of about 50-100 ohm-cm were used inthe current study. Both highly branched nanoporous and hybrid nanoporouscovered microporous PS substrate samples were fabricated in anelectrochemical cell constructed from high-density polyethylene. Theworking electrode was attached to the back of a p-type silicon wafer<100> (aluminum coated) and the counter electrode corresponded to aplatinum foil placed in solution. The cell was sealed to the front ofthe wafer, using a clamp, as about a 1 cm² section of the wafer madecontact with the solution. A magnetic stir-bar was used to prevent thebuild-up of hydrogen at the surface of the silicon. The electrochemicaletching current was supplied by an Potentiostat/Galvanostat. Thenanoporous samples were etched in an aqueous 25% HF in methanol solutionwhile the hybrid samples were etched in a solution of 1 M H₂O, 1 M HF,and 0.1M tetrabutylammonium percholate (TBAP), all in acetonitrile. Theaqueous etched samples were etched at a current density ranging fromabout 5 to 6 mA/cm² for about 50 to 75 minutes while the hybrid sampleswere etched with a current density of about 6 mA/cm² for between 50 and75 min. Using this latter procedure, pores approximately 1 to 2 μm wideby about 10 μm deep were formed, and well covered by a coating ofnanoporous silicon.

[0042] Further, as discussed in example 1, single-crystal <100>,boron-doped silicon wafers (substrate) of resistivity ranging from about1 to about 50 ohm cm were also etched in an aqueous HF solution. Forseveral of the experiments (about 20% concentration of HF in methanol),a 300-nm thin film of aluminum was sputtered onto the backside of thewafers. Electrical connections were made to the wafers by connecting awire to the thin film of aluminum using conductive paint. The wire andaluminum film were then covered with a layer of black wax, leaving onlythe front surface of the silicon exposed to the etching solution. Boththe wired silicon-wafer as one electrode and a platinum wire as acounter-electrode could be connected through a Teflon cap, which wastightly fitted to a cuvette containing the etch solution. Ohmic contactswere made to the wafer by connecting a wire to the thin film of aluminumusing conductive paint. Etching currents ranged from about 2 to 30mA/cm², but the samples considered here were usually etched at 8 mA/cm²for 10 minutes.

[0043] After the aqueous and hybrid etches were complete, the sampleswere removed to air, washed with methanol, and dried. For the majorityof the samples, the etched PS substrate samples were treated withanhydrous concentrated hydrazine (about 30 M) to remove fluorine fromthe surface via a reaction which converts the fluorine and hydrazine tonitrogen and HF. Several of the samples were later treated with a 6MHCl/MeOH solution to: (1) enhance the photoluminescence and (2)stabilize the photoluminescence quantum yield.

[0044] An electroless copper solution was prepared, following standardprocedures known in the art, from CuSO₄.5H₂O (0.76 g), sodium potassiumtartrate (4.92 g), formaldehyde (2 ml), and NaOH (0.80 g) diluted to 200ml in doubly de-ionized water. A slightly modified procedure was used toprepare an electroless silver solution. First 0.75 M/L NH₄NO₃ (12 g/200ml) and 2 M/L NH₃ (24.3 mL NH₄OH/200 ml H₂O) solutions were mixedtogether and diluted in a 100 mL volumetric flask with doubly ionizedwater. To this solution was added 0.09 M/L AgNO₃ (1.36 g AgNO₃/200 ml)followed by 0.1 M/L Co (NO₃)₂.6H₂O (5.82 g diluted into 50 ml of doublyde-ionized water). To this mixture was added sufficient doublyde-ionized water to bring the total solution volume to 200 ml. Thecopper and silver solutions were maintained in a refrigerator at 20° C.until they were used. As discussed above, other metal-containingsolutions can be produced and used.

[0045] Both the nanoporous and hybrid macroporous-nanoporous PS sampleswere exposed to the copper and silver electroless solutions eitherdirectly or after the samples were treated with HCl. The majority of thesamples were also treated with anhydrous hydrazine. Samples were exposeddirectly to the electroless solutions both under ultraviolet/visible(uv) and HeNe laser PLE, in complete darkness, or in the presence oflaboratory room lights. The observation of the reduction of theelectroless solution metallic ions and the subsequent metal depositionwas found to be illumination dependent.

[0046] The nature of the contact to the PS surface formed from <100>p-type silicon is now discussed. To place electroless metal contacts onthe PS surface, this surface was first passivated with anon-stoichiometric silicon nitride, SiN_(x), layer which was grown byplasma enhanced chemical vapor deposition, PECVD. The openings in theSiN_(x) layer needed for formation of the porous layer were made byreactive ion etching, RIE. After an anodic etch, the porous layer wasprepared and electroless copper (Cu) was deposited on the surface ofthis PS substrate layer to form the metal contacts necessary to make aresistance measurement.

[0047] Data was obtained for PS substrate samples exposed to electrolesssilver (Ag) and copper solutions at about 16 and 30° C. For the vastmajority of these experiments the samples were bathed in anhydroushydrazine for periods of 30, 60, or 90 minutes before exposure in theelectroless solutions. This treatment was carried out to remove fluorinebased constituents from the PS substrate surface. With the removal offluorine, the effects of optical pumping as it produces the long-livedphotoluminescent emitter, can be readily evaluated. PS substratesamples, which are photoluminescent, are capable of plating silver andcopper from an electroless solution, in which they are in contact, ifexposed to uv/visible light or when exposed to a HeNe laser. If the PSsurface is not photoluminescent or the PL from a photoluminescentsurface is quenched, the deposition of copper or silver is diminished orcompletely absent. While temperature is an important consideration inthese experiments, measurements of the very small surface temperaturechange as a function of exposure to the light sources used in this studydemonstrate that the metal plating is not the result of a surfaceheating effect. However, an increased ambient temperature for the PSsubstrate surface, especially for those experiments involvingelectroless silver plating, can lead to an enhancement of the depositionprocess. In other words, the plating is more pronounced and moredifficult to control precisely at 30° C. than at 16° C.

[0048] Photoluminescent PS substrate samples were clamped to the surfaceof a hollow copper block whose temperature could be adjusted by flowingwater through a slush bath configuration at room or ice temperatures.Simultaneously, the sample under study was placed under a ring used tohold the electroless solution as the entire experiment was carried outunder a flowing stream of high purity argon. Over the course of anexperiment, the surface temperature was measured with a thermocouple.While this system could be operated at temperatures considerably lowerthan the ice bath temperature, T_(surface) (measured)=16±1° C., it wasfound that: (1) this was unnecessary to control the electroless process;and (2) that colder device temperatures eventually led to theundesirable condensation of the electroless solution. The temperaturerise associated or induced by the high intensity uv/visible lamp, neverexceeded 1.5° C. No temperature rise was recorded for those experimentswith the HeNe laser (even if the laser was focused onto the tip of thethermocouple).

[0049] The reduction of silver from the electroless solution was foundto be considerably more efficient than was that of copper from itselectroless solution. For the silver samples at 16° C., hydrazineexposure was systematically varied from 30 to 60 to 90 secondsdemonstrating only a moderate effect with increased exposure on theobserved plating. Samples treated with hydrazine plate at a much slowerrate than do untreated samples which have maintained a fluorideconstituency on the PS substrate surface. Results were obtained for bothaqueous etched and hybrid etched samples, the latter interacting withthe electroless solutions notably more effectively.

[0050] SEM micrographs of copper and silver deposition into the pores ofa hybrid macroporous nanoporous sample at 16° C. demonstrate thedeposition of metal to the walls of the micropores. The results of theresistance measurements on the electroless copper connections indicateresistances ranging from about 20 ohms to about 1000 ohms.

[0051] The electroless copper solution used in these experiments isconsiderably more stable (kinetically) than the electroless silversolution. In contrast to a freshly formed PS substrate surface, there isno plating on a c-Si substrate from the electroless copper solution andlikewise there is no plating on PS substrate samples which have beenoxidized in air for periods exceeding several weeks. As a freshly formedphotoluminescent sample readily plates copper at room temperature, theplating ability of a PS substrate sample which has been subjected toextended oxidation can be restored with a brief exposure to a HClsolution. Samples which are exposed to the electroless copper bath willgenerally not plate copper if they: (1) are placed in a darkenedlocation; (2) are soaked in etching solution in the absence of currentflow before exposure; (3) are a heated at their surface in the presenceof the electroless bath; or (4) have undergone a previous PL quenchingprocess do not plate copper.

[0052] Thus, the plating process appears to require, at least in part,that a treated PS substrate surface be photoluminescent. However, it isknown in the art that copper can be reductively deposited, in smallconcentration, on a PS substrate surface from an aqueous Cu⁺² solutionin the absence of photoexcitation. Thus, anhydrous hydrazine has beenintroduced to modify the initially prepared PS substrate as a means ofremoving the fluoride centers on the surface, which may act as reducingcenters for Cu⁺² (aq) ions. Upon treatment with hydrazine the copperreduction at the PS substrate surface (1) is slowed to an extent whichallows a significant improvement in the degree of control of themetallization process, (2) clearly becomes a function of surface excitedstate fluorophors, and (3) is notably more amenable to patternformation. The exposure of an untreated PS substrate surface, with itssurface fluoride constituency, to the HeNe laser produces aphotodefineable pattern in 5 minutes whereas the pattern produced with aXe ArC lamp, with a plating time approaching one minute, is notphotodefineable. Surface fluoride, like palladium, appears to act as acatalyst for the reduction process.

[0053] The PS substrates, whose luminescence has been quenched, cannotbe patterned by the present method. While the mechanism of electrolessdeposition onto PS might result in part from electron-hole pairgeneration within the PS substrates it more likely results from excitedstate electron transfer involving surface-confined silicon oxyhydrides.

[0054] C. Dye Enhanced PL of PS Substrates

[0055] Room temperature PL of PS substrates has attracted considerableattention primarily because of its potential use in the development ofsilicon based optoelectronics, displays, and sensors. However, therelatively long excited state lifetime associated with this PL, whichappears to be of the order of tens to hundreds of microseconds, isproblematic for some of these applications. The efficiency andwavelength range of the emitted light can be affected by the physicaland electronic structure of the surface, the nature of the etchingsolution, and the nature of the environment into which the etched sampleis placed.

[0056] These outlined results suggest the possibility that severalcommon fluorescent dyes, whose radiative lifetimes are of the order ofnanoseconds, might be made to interact with the PS substrate surface soas to considerably improve the observed PL rate. Strong physisorption orchemisorption of certain of these fluorescent dyes with the fluorescentemitter on the PS substrate surface through complexation enhance thequantum yield and modify the lifetime of the fluorescent eventsassociated with the PS substrate.

[0057] More specifically, the post-etch enhancement of the PL from a PSsubstrate, which has a microporous framework on which is superimposed ananoporous layer, includes treating the PS substrate with a dye. Thedyes that can be used include, but are not limited to,3,3-diethyloxadicarbacyamine iodide, Rhodamine dye compounds (e.g.Rhodamine 6G, Rhodamine 700), Fluorocein, dicyanomethylene (DCM) dyecompounds (e.g.4-dicyanomethylene-2-methyl-6-(p-dioctylaminostyryl)-4H-pyran,4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran,4-dicyanomethylene-2-methyl-6-[2-(1-methyl-1,2,3,4-tetrahydroquinolin-6-yl)ethenyl]-4H-pyran),4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-8-yl)etheny]-4H-pyran), and other dyes that havenegligible absorption at the wavelengths of maximum absorption for thePS substrate.

EXAMPLE 3

[0058] The following is a non-limiting illustrative example of anembodiment of the present invention and is described in more detail inGole, et al., J. Phys, Chem. B, 103, 979 (1999), which is incorporatedherein by reference. This example is not intended to limit the scope ofany embodiment of the present invention, but rather is intended toprovide specific experimental conditions and results. Therefore, oneskilled in the art would understand that many experimental conditionscan be modified, but it is intended that these modifications are withinthe scope of the embodiments of this invention.

[0059] As discussed for example 1 and 2, single crystal <100> borondoped silicon wafers with resistivities of about 50-100 ohm-cm were usedin the current study. Both highly branched nanoporous and hybridnanoporous covered microporous PS substrate samples were fabricated inan electrochemical cell constructed from high-density polyethylene. Theworking electrode was attached to the back of a p-type silicon wafer<100> (aluminum coated) and the counter electrode corresponded to aplatinum foil placed in solution. The cell was sealed to the front ofthe wafer, using a clamp, as about a 1 cm² section of the wafer madecontact with the solution. A magnetic stir-bar was used to prevent thebuild-up of hydrogen at the surface of the silicon. The electrochemicaletching current was supplied by an Potentiostat/Galvanostat. Thenanoporous samples were etched in an aqueous 25% HF in methanol solutionwhile the hybrid samples were etched in a solution of 1 M H₂O, 1M HF,and 0.1M tetrabutylammonium percholate (TBAP), all in acetonitrile. Theaqueous etched samples were etched at a current density ranging fromabout 5 to 6 mA/cm² for about 50 to 75 minutes while the hybrid sampleswere etched with a current density of about 6 mA/cm² for between 50 and75 min. Using this latter procedure, pores approximately 1 to 2 μm wideby about 10 μm deep were formed, and well covered by a coating ofnanoporous silicon.

[0060] Further, as discussed in example 1, single-crystal <100>,boron-doped silicon wafers (substrate) of resistivity ranging from about1 to about 50 ohm cm were also etched in an aqueous HF solution. Forseveral of the experiments (about 20% concentration of HF in methanol),a 300-nm thin film of aluminum was sputtered onto the backside of thewafers. Electrical connections were made to the wafers by connecting awire to the thin film of aluminum using conductive paint. The wire andaluminum film were then covered with a layer of black wax, leaving onlythe front surface of the silicon exposed to the etching solution. Boththe wired silicon-wafer as one electrode and a platinum wire as acounter-electrode could be connected through a Teflon cap, which wastightly fitted to a cuvette containing the etch solution. Ohmic contactswere made to the wafer by connecting a wire to the thin film of aluminumusing conductive paint. Etching currents ranged from about 2 to 30mA/cm², but the samples considered here were usually etched at 8 mA/cm²for 10 minutes.

[0061] Prepared aqueous and hybrid samples were removed from the etchingsolution, washed in reagent grade methanol and treated with 10⁻³ molar(in doubly distilled H₂O) 3,3′-diethyloxadicarbocyanine iodide (DODCl)or Rhodamine 700. The PS substrate samples were dipped for severalseconds or soaked for periods extending to about 45 minutes.

[0062] Pore structure and PL emission from PS substrates for untreatedPS substrate samples is now discussed. The PL emission from aqueous andhybrid etched samples observed over the period 1.5-100 μs afterexcitation (PLE) at 337.1 nm (t=0) suggests that the PL observed for thehybrid etched sample exceeds that for the aqueous etched sample anddemonstrates a slight 10-15 nm red shift.

[0063] PS substrates are known in the art to display a “green” PLresulting from an intermediate precursor state in the earlier stages ofits formation, especially in an aqueous etch solution. The temporaldecay and spectral profile of the “green” PL and transformation to afinal “orange-red” PL emission during and following PS formation suggestthe coupling of these PL emitters to the PS surface. The manifestationsof the green and orange-red emission features in spectra are virtuallyidentical for the aqueous and hybrid etched samples. Observed spectrumhistograms represent the first clear observation of the “green”luminescence feature in an air-aged sample and demonstrate the magnitudeof its contribution to the overall spectrum. With time (1) the “green”and “orange-red” emission features merge into each other as the sourceof the green emitter undergoes oxidative transformation to the final“orange-red” emitter, and (2) the longer wavelength featurescontributing initially to the orange-red emission are seen to decay mostrapidly leading to what appears to be the manifestation of a blue shiftin this feature in the absence of etching.

[0064] After 5.5 μs, for the aqueous etched sample, and 9.5 μs for thehybrid etched sample, the observed spectral features change little withdelay time and the emission signal over the gate width of the scanbegins to decrease. The dominant characteristics of the 1.5-100 μsspectra develop over the time span of the histograms, and with longertime delays, the monitored emission, while maintaining an identicalwavelength dependence, decreases precipitously in intensity. The datademonstrate a nearly parallel although slightly different development ofthe PL intensity for the aqueous and hybrid etches and an overallspectral distribution which is quite similar for these etched samples.

[0065] Dye-treated PS substrates are now discussed. Common dyes, such asDODCl and Rhodamine 700, have been used because of their negligibleabsorbance in the 350±20 nm range, the approximate peak absorption rangeof the PS excitation spectrum. The absorption spectrum for DODCldemonstrates a minimal absorbance for λ=330-470 nm. Rhodamine 700 isalso a reasonable candidate although this dye does display a smallabsorbence at λ<330 nm.

[0066] Here, the focus is to create an environment for the energytransfer pumping of the adsorbed dye and/or the mediation of thelonger-lived fluorescence from the PS substrate due to PS-dyecomplexation. As the optical pumping of the PS substrate surface isknown to access a long-lived excited state triplet exciton, this excitedstate can be an energy reservoir for subsequent energy (or electron)transfer between the PS substrate surface and the adsorbed dye. Suchtransfer might take place through the pumping of a dye molecule in closeproximity to the PS substrate surface via a fast intermolecular electrontransfer. Alternatively, the dye chemisorbed with the surface-activeexciton could receive the exciton energy via fast intramolecular energytransfer along a short bonding chain. Finally, the presence of this muchmore efficient radiator could enhance the PS substrate emission ratesimply through complexation with the PS fluorophors.

[0067] Samples exposed to DODCl or Rhodamine 700 dye, when pumped at337.1 nm by a nitrogen laser, display an initial quenching of the PSsubstrate PL followed by an expected slow and continued increase in thePL emission rate upon aging in the dark, in air, for an extended period.The aging cycle eventually produces a PL signal which has been maximizedand maintained for a period of several months.

[0068] After some period of aging, the DODCl treated sample is found todisplay a photoluminescence corrected for phototube and system responsewhich exceeds the intensity of a nominally prepared PS substrate sampleby a factor of five. Further, the distribution of fluorescence isnotably broader with a peak response considerably red shifted (about30-50 nm) from an aqueous etched untreated PS substrate sample. This isconsistent with a PS-dye coupling. Note that, as opposed to hightemperature annealing at temperatures between 100° C. and a verysignificant 600° C. for short periods to promote oxidation on the PSsubstrate surface, this embodiment shows a long-term aging process underconditions which promote the conversion of the surface and ensure a dyeinitiated modification without seriously modifying the interactingconstituencies.

[0069] Simple immersion or prolonged soaking of the PS substrates inmillimolar (mM) dye solutions is sufficient not only to position the dyein close proximity to the surface bound PS emission centers but also topromote its interaction. The most pronounced interaction is manifest forthose samples treated in DODCl. The effects observed for both DODCl andRhodamine 700 are greatest for those samples treated after aqueousetching.

[0070] A histogram with delays ranging from 0.5 to 61.5 μs (5 μs gate)for the DODCl treatment of an aqueous etched sample shows a convergenceto a dominant feature peaking at about 650 nm. By comparison, ahistogram of the PL for time delays of 0.5 to 11.5 μs (5 μs gate)demonstrates the evolution observed for an untreated aqueous etchedsample. The DODCl treated sample is distinct, displaying initially botha “green” emission feature and an “orange-red-red” emission featurewhich at first appear to “bookend” the observed aqueous etch emissionfeatures. With increased delay time, the green emission feature redshifts and the red emission feature appears to split into two features,one of which blue shifts with increased delay time and a second peakwhich appears almost stationary in time. This gives the appearance of atriple peaked spectrum for time delays ranging from 7.5 to 23.5 μs.After a 27.5 μs delay, the shifting short wavelength and orange-redfeatures have virtually merged into each other to form a dominant peakat about 640 nm which eventually red shifts by about 10 nm. The observedspectra after 27.5 μs suggest that the continued red shifting of theinitially green emission feature somewhat dominates the observed timedependence. The peak spectral intensity observed in a given histogramremains virtually constant out to 33.5 μs.

[0071] The interaction of Rhodamine 700 with an aqueous etched PSsubstrate sample appears to be much less pronounced than that for DODCl.The shifting green and orangered emission features have merged in the7.5 μs delay scan to a dominant spectral peak at 640 nm which red shiftsto 650 nm by 21.5 μs. Further, the drop off in spectral intensity occursconsiderably more rapidly.

[0072] In another histogram obtained for a DODCl treated hybrid etchedsample, initially soaked for 45 minutes in a 10⁻³ M dye solution, thedye has a clear affect on the hybrid etched sample although not aspronounced as that on the aqueous etched sample. Further, the extendedperiod of exposure to the dye is required as a hybrid sample simplydipped in DODCl is found to rapidly converge to a strongly dominant630-640 nm feature indicative of the untreated hybrid etch. In anotherhistogram, the PL for the DODCl treated sample is compared to theevolution for an untreated hybrid etch sample for time delays of 0.5 to9.5 μs (5 μs gate). The histograms display the cycle of convergence forthe red shifting “green” and blue shifting “orange-red” emissionfeatures. However, the appearance of the triple peaked spectrum(τ_(delay)˜3.5 μs) and the time delay corresponding to the merging ofthe shorter wavelength and orange-red emission features to a dominant630 nm (peak) feature, ˜11.5 s, occur on a considerably shorter timescale. The spectral intensity of the DODCl treated sample begins to dropoff rapidly for time delays longer than 17.5 μs, converging to a finalpeak wavelength for the dominant feature at 650 nm.

[0073] A further histogram for a Rhodamine 700 treated hybrid etchedsample suggests that even a 45 minute exposure has only a small effect.In fact, the convergence of the spectral features to a dominant singlepeak appears to occur even more rapidly than the untreated sample overthe range of delay times 3.5 μs or less. A significant drop off inspectral intensity is observed to occur for time delays longer than 7.5μs.

[0074] It should be emphasized that the above-described embodiments ofthe present invention, particularly, any “preferred” embodiments, aremerely possible examples of implementations, merely set forth for aclear understanding of the principles of the invention. Many variationsand modifications may be made to the above-described embodiment(s) ofthe invention without departing substantially from the spirit andprinciples of the invention. All such modifications and variations areintended to be included herein within the scope of this disclosure andthe present invention and protected by the following claims.

Therefore, having thus described the invention, at least the followingis claimed:
 1. A post-etch method of enhancing and stabilizing thephotoluminescence from a porous silicon substrate, comprising the stepsof: treating the porous silicon substrate with an aqueous hydrochloricacid solution; and treating the porous silicon substrate with analcohol.
 2. The method of claim 1, wherein the step of treating theporous silicon substrate includes treating a porous silicon substratethat has a microporous framework on which is superimposed a nanoporouslayer.
 3. The method of claim 1, wherein the step of treating the poroussilicon substrate with an alcohol further comprises the step of treatingthe porous silicon substrates with methanol.
 4. The method of claim 1,wherein the step of treating the porous silicon substrate with thedilute aqueous hydrochloric acid further includes a step of treating theporous silicon substrate with at least a 0.2 molar aqueous hydrochloricacid solution.
 5. A post-etch method of enhancing and stabilizing thephotoluminescence from a porous silicon substrate, comprising the stepof treating the porous silicon substrate with an aqueous hydrochloricacid and alcohol solution.
 6. The method of claim 5, wherein the step oftreating the porous silicon substrate includes treating a porous siliconsubstrate that has a microporous framework on which is superimposed ananoporous layer.
 7. The method of claim 5, wherein the step of treatingthe porous silicon substrate with the dilute aqueous hydrochloric acidand alcohol solution further includes the step of treating the poroussilicon substrate with an aqueous hydrochloric acid and methanolsolution.
 8. The method of claim 5, wherein the step of treating theporous silicon substrate with the dilute aqueous hydrochloric acid andalcohol solution further includes a step of treating the porous siliconsubstrate with at least a 0.2 molar aqueous hydrochloric acid andalcohol solution.
 9. A post-etch method of electroless metallization ofa porous silicon substrate, comprising the steps of: treating the poroussilicon substrate with an aqueous hydrochloric acid solution; treatingthe porous silicon substrate with an alcohol; treating the poroussilicon substrate with a hydrazine solution to remove fluorides from theporous silicon; introducing a metal-containing electroless solution;illuminating the porous silicon substrate with a light source atwavelengths less than about 750 nanometers to cause photoluminescence ofthe porous silicon substrate; and metallizing the porous siliconsubstrate wherein photoluminescence is capable of causing metallizationof the porous silicon substrate with the metal of the metal-containingsolution.
 10. The method of claim 9, wherein the step of treating theporous silicon substrate includes treating a porous silicon substratethat has a microporous framework on which is superimposed a nanoporouslayer.
 11. The method of claim 9, wherein the step of treating theporous silicon substrate with an alcohol further includes the step oftreating the porous silicon substrates with methanol.
 12. The method ofclaim 9, wherein the step of treating the porous silicon substrate withthe dilute aqueous hydrochloric acid further includes a step of treatingthe porous silicon substrate with at least a 0.2 molar aqueoushydrochloric acid.
 13. The method of claim 9, wherein the step ofilluminating the porous silicon substrate with the light source atwavelengths less than 750 nanometers further includes the step ofilluminating the porous silicon substrate with a Xenon arc lamp lightsource at wavelengths less than 750 nanometers.
 14. The method of claim9, wherein the step of illuminating the porous silicon substrate withthe light source at wavelengths less than 750 nanometers furtherincludes the step of illuminating the porous silicon substrate with aHelium Neon laser light source at wavelengths less than 750 nanometers.15. A post etch method of electroless metallization of porous siliconsubstrate, comprising the steps of: treating the porous siliconsubstrate with an aqueous hydrochloric acid and alcohol solution;treating the porous silicon substrate with a hydrazine solution toremove fluorides from the porous silicon; introducing a metal-containingsolution; illuminating the porous silicon substrate with a light sourceat wavelengths less than about 750 nanometers to cause photoluminescenceof the porous silicon substrate; and metallizing the porous siliconsubstrate wherein photoluminescence is capable of causing metallizationof the porous silicon substrate with the metal of the metal-containingsolution
 16. The method of claim 15, wherein the step of treating theporous silicon substrate includes treating a porous silicon substratethat has a microporous framework on which is superimposed a nanoporouslayer.
 17. The method of claim 15, wherein the step of treating theporous silicon substrate with the dilute aqueous hydrochloric acid andalcohol solution further includes the step of treating the poroussilicon substrate with an aqueous hydrochloric acid and methanolsolution.
 18. The method of claim 15, wherein the step of treating theporous silicon substrate with the dilute aqueous hydrochloric acid andalcohol solution further includes a step of treating the porous siliconsubstrate with at least a 0.2 molar aqueous hydrochloric acid andalcohol solution.
 19. The method of claim 15, wherein the step ofilluminating the porous silicon substrate with the light source atwavelengths less than 750 nanometers further includes the step ofilluminating the porous silicon substrate with a Xenon arc lamp lightsource at wavelengths less than 750 nanometers.
 20. The method of claim15, wherein the step of illuminating the porous silicon substrate withthe light source at wavelengths less than 750 nanometers furtherincludes the step of illuminating the porous silicon substrate with aHelium Neon laser light source at wavelengths less than 750 nanometers.21. The method of claim 15, further including the step of forming ametallized porous silicon substrate that has a resistance of about 20ohms to about 1000 ohms.
 22. The method of claim 15, further includingthe step of forming a metallized porous silicon substrate that has aresistance of about 20 ohms to about 500 ohms.
 23. The method of claim15, further including the step of forming a metallized porous siliconsubstrate that has a resistance of about 20 ohms to about 60 ohms.
 24. Asubstrate comprising a metallized porous silicon substrate.
 25. Thesubstrate of claim 24 further including a copper metallized poroussilicon substrate.
 26. The substrate of claim 24 further including asilver metallized porous silicon substrate.
 27. The substrate of claim24, wherein the metallized porous silicon substrate has a resistance ofabout 20 ohms to about 1000 ohms.
 28. The substrate of claim 24, whereinthe metallized porous silicon substrate has a resistance of about 20ohms to about 500 ohms.
 29. The substrate of claim 24, wherein themetallized porous silicon substrate has a resistance of about 20 ohms toabout 60 ohms.
 30. A post etch method of enhancing photoluminescencefrom a porous silicon substrate that has a microporous framework onwhich is superimposed a nanoporous layer, comprising the step oftreating the porous silicon substrates with a dye, wherein the dye isselected from the group consisting of: Rhodamine dye compounds;3,3′-diethyloxadicarbocyanine iodide; Fluorocein; and dicyanomethylenedye compounds.
 31. The method of claim 30, wherein the porous siliconsubstrate has a microporous framework on which is superimposed ananoporous layer.